The present invention relates to a dynamic pressure bearing apparatus applicable to a motor for a hard disk drive as well as a method for manufacturing such dynamic pressure bearing apparatus. More particularly, the present invention related to a dynamic pressure bearing apparatus used to increase the rotational accuracy of a hub to which a rotational body such as a disk is mounted.
As is known, a dynamic pressure bearing apparatus includes a shaft and a sleeve which rotate in relation to each other. As the apparatus starts rotating at a high speed, dynamic pressure is generated by the dynamic pressure generating grooves that are formed, for example, on the bearing surface. The rotor rotates with high accuracy due to the stiffness of the bearing. By applying such an apparatus as a bearing apparatus in a motor for a hard disk drive, high recording density can be obtained. However, further improvement in recording density is demanded and therefore an increase in rotational accuracy is desired.
A conventional method for manufacturing a dynamic pressure bearing apparatus is described with reference to FIGS. 6 and 7. As shown in FIG. 6, a ring-shaped thrust plate 22 is fixed to the outer circumference at the bottom of rotatable shaft 21 to form a shaft assembly by integrating the two parts. Then, rotatable shaft 21 is inserted through the center hole of sleeve 23 from the bottom. Thrust plate 22 fits in a concavity at the bottom of sleeve 23 with a given space. Counter plate 24 is engaged into another concavity having a diameter larger than the previous concavity at the bottom of sleeve 23. Counter plate 24 is fixed to sleeve 23, and an adhesive is used for sealing such that a sleeve assembly is provided.
Rotatable shaft 21 is rotatable in relation to sleeve 23 wherein a bearing assembly is formed by inserting oil as a lubricant fluid between rotatable shaft 21 and sleeve 23. Dynamic pressure generating grooves are formed on at least one of the outer surface of rotatable shaft 21 and the inner surface of sleeve 23. A dynamic pressure bearing is formed by placing the oil between the dynamic pressure generating grooves, rotatable shaft 21 and sleeve 23.
On the other hand, a core assembly is formed by winding drive coil 26 around each of projecting poles of laminated core 25. A center hole of laminated core 25 of the core assembly is engaged to the outer circumference of sleeve 23 of the bearing assembly and is fixed thereto by an adhesive. Oil absorbent fabric 27 is placed on the laminated core from the upper end of rotatable shaft 21 and cover 28 is engaged thereto. As shown in FIG. 7, the laminated core is mounted to the upper surface of sleeve to form a stator assembly.
Further, in a separate process, as shown in FIG. 7, cylindrical rotor magnet 33 is engaged to the inside ceiling of hub 30, with a reversed-cup shape, via seal 32 to form a rotor assembly. Rotor magnet 33 is magnetized, alternating the N and S poles with a given distance, in a circumferential direction. It is fixed to the inner surface of hub 30 with an adhesive or the like. Hub 30 has disk mounting surface 31 which projects as a flange to the outer circumference. The rotor assembly is placed on the top of the stator assembly to form a motor assembly by fixing the top end of rotatable shaft 21 to the center hole hub 30.
The center hole of base 34 is engaged to the outer circumference at the bottom of sleeve 23, forming the motor assembly, and is fixed with an adhesive and the like. Insulator 29 is placed between the stator portion of the motor assembly and base 34. Flexible print wiring substrate 35 is placed at a concavity formed on the bottom surface of base 34. Each drive coil 26 and a feeder circuit are connected by flexible print wiring substrate 35.
A current flow to each drive coil 26 is controlled while detecting the rotational position of rotor magnet 33, causing magnetic attraction / repulsion between rotor magnet 33 and the projecting pole of the stator. As a result, a force is applied to the rotor, which is formed together with rotor magnet 33, in the circumferential direction such that the rotor rotates. Dynamic pressure is generated, by rotation of rotatable-shaft 21 as a part of the rotor, between the outer surface of rotatable shaft 21 and the inner surface of sleeve 23 due to viscous resistant of the oil. Then, rotatable shaft 21 is supported in the radial direction without having its outer surface mechanically contact the inner surface of sleeve 23. Also, a thrust dynamic pressure bearing is formed between thrust plate 22, sleeve 23 and counter plate 24; the thrust load of rotatable shaft 21 is supported without mechanically contacting the other parts.
According to a conventional method for manufacturing a dynamic pressure bearing apparatus as described above, hub 30 and rotatable shaft 21 are assembled to each other at the end of the manufacturing process. Therefore, one may not be able to obtain high accuracy of disk mounting surface 31 of hub 30 for vibration or deviation in the axial direction in relation to the rotational center of rotatable shaft 21. This is due to the space between rotatable shaft 21 and sleeve 23, which is sufficient for generating dynamic pressure; because when hub 30 is fixed to rotatable shaft 21 at the end of the manufacturing process, rotatable shaft 21 may be assembled to hub 30 with leaning by an amount of the space therein.
Also, the length of a shaft in the axial direction tends to be shortened along with a decrease in the thickness of apparatus in recent years. In the case of a short length in the axial direction with a thin bearing, according to the conventional method for manufacturing a dynamic pressure bearing apparatus, sufficient length for connecting hub 30 and rotatable shaft 21 cannot be obtained. This is another reason why it is difficult to provide accuracy of disk mounting surface 31 for vibration in the axial direction.
Further, in the case of a short length in the axial direction with a thin bearing, it is difficult to obtain sufficient length for connecting hub 30 and rotatable shaft 21. As a result, bonding strength is not sufficient, when impact is applied, the product is deformed such that desired characteristics are not obtained.
When accuracy of disk mounting surface 31 for vibration or deviation in the axial direction is not sufficiently obtained, in general, after hub 30 is fixed to rotatable shaft 21, then disk mounting surface 31 of hub 30 is cutting processed to reduce vibration or deviation while hub 30 is rotated. However, resolving the problem of vibration or deviation by cutting while rotating with an outside drive after assembling the dynamic pressure bearing as described above causes shortening of the life of the bearing due to contacts among parts thereof.
It therefore is an object of the present invention to provide a dynamic pressure bearing apparatus and a method for manufacturing thereof in which vibration of a hub can be recognized and controlled in the first stage of an assembly process such that accuracy therein is increased and the yield is increased for a lower cost.
It is a further object to provide a dynamic pressure bearing apparatus and a method for manufacturing thereof in which oil as a lubricant fluid can be easily inserted and oil leakage is minimized.
To achieve the above objects, a dynamic pressure bearing apparatus is provided in which a hub is fixed to a rotatable shaft and a disk mounting surface of the hub is processed, wherein the rotatable shaft is inserted into a sleeve. The apparatus is provided with high accuracy as compared to conventional technology in which the vibration of the hub is adjusted at the end of the assembly process. Yield is improved, and the accuracy is guaranteed in the pre-process so that assembly errors and manufacturing cost can be decreased.
In accordance with aspect of the invention, a space between a hub and the end surface of a sleeve is established larger than the bearing space of the dynamic pressure bearing, and is formed to widen outwardly in relation to the side of the dynamic pressure bearing. Therefore, oil can be readily inserted through the relatively large space to the dynamic pressure bearing. Also, since the space between the hub and the end surface of the sleeve is formed to widen outward, cleaning after insertion of the oil is made easy.